1. Field of the Invention
The present invention relates to a gas dynamic pressure bearing, and more particularly to a gas dynamic pressure bearing employed in a spindle motor, a recording disk drive device, and a polygon scanner.
2. Background Information
There have been demands in recent years for increases in the rotational frequency of data recording devices such as recording disk drive devices and optical disk devices used in computers and the like. In addition, there have also been demands for polygon scanners used in digital copiers, laser printers, and the like to achieve high rotational speeds in the range of 20,000 rpm or more in order to improve print quality and to accelerate the devices that use them. The use of gas dynamic pressure bearings in the motors employed by the aforementioned devices has been proposed in order to allow the motors used therein to achieve both high rotational speeds and an extended life span.
However, the following problem has been identified with gas dynamic pressure bearings. If a gas dynamic pressure bearing does not rotate at or above a certain speed, there will not be enough dynamic pressure generated therein to prevent the bearing surfaces from coming into contact with each other. Because of this, there is a tendency to increase the torque needed for rotation during startup until the motor achieves its rated speed. Abrasion of the bearing surfaces cannot be ignored because the thrust bearing surfaces come into direct contact with each other, particularly during the initial phase of startup.
Various methods have been attempted in order to solve this problem. For example, Japanese Published Patent Application H09-318900 discloses a method of maintaining a space between the bearing surfaces when the motor is stopped, and thus making startup easier, by providing a microscopic convex portion on the bearing surfaces.
However, this method requires that the convex portion be formed with a high degree of precision. In other words, if the convex portion is too low, the surface thereof come into contact with other concave and convex portions on the bearing surfaces and thus startup torque cannot be reduced. On the other hand, if the convex portion is too high, the motor will continue to rotate with the convex portions in contact with each other, thereby severely abrading the convex portion. Thus with this method, it is difficult to reduce production costs, and there are concerns that the reliability of the bearing will be harmed due to variations in the precision thereof.
In addition, Japanese Published Patent Application 2000-87958 discloses a method which provides a convex portion on the inner circumferential side of the thrust plate in order to allow the bearing surfaces to avoid contacting each other when the motor is stopped. This method also forms a concave portion that is more shallow than the dynamic pressure generating grooves thereon, and an elevated portion that is lower than the convex portion, on the outer circumferential side of the convex portion in order to accelerate the generation of dynamic pressure when the motor starts up.
In this method, the degree of precision that the concave portion requires is less than that of the method disclosed in Japanese Published Patent Application H09-318900. However, this method increases the number of portions which require precision, such as the concave portion that is more shallow than the dynamic pressure generating grooves on the outer circumferential side of the convex portion, and the elevated portion that is lower than the convex portion. Because of this, it is difficult to reduce the cost of producing the bearing, and there are concerns that the reliability of the bearing will be harmed due to irregularities in the precision thereof.
Furthermore, Japanese Published Patent Application 2000-304037 discloses a structure that is comprised of a cylindrical structure that is coaxially inserted into a shaft, and a cylindrical sleeve that is closed on both ends thereof and which accommodates and nests the cylindrical structure therein such that it is freely rotatable therewith. Thrust dynamic pressure bearings are formed in the closing surfaces of the sleeve that face both end surfaces of the cylinder, and a radial dynamic pressure bearing is formed in the inner circumferential surfaces of the sleeve that face the lateral surfaces of the cylinder. In this structure, a gas path is provided in the portions thereof that connect the gap in the thrust dynamic pressure bearings with the gap in the radial dynamic pressure bearing, which stabilizes the rotation of the motor at low speeds.
However with this method, it is difficult to increase the rigidity of the bearing in the radial direction, and thus it is difficult to achieve the bearing performance required in a recording disk drive device and the like.
Up until now, no gas dynamic pressure bearing has been capable of satisfying demands that (a) the bearing surfaces thereof promptly move to the non-contact state at startup, (b) the manufacture thereof be simplified, and (c) the bearing have a high degree of rigidity.